The present invention relates to a bipolar transistor device and to a method for fabricating the same. More particularly, it relates to a heterobipolar transistor device using a semiconductor containing silicon (Si) and a semiconductor containing a gallium nitride (GaN) and to a method for fabricating the same.
FIG. 9A shows a cross-sectional structure of a conventional embodiment of a so-called SiGe heterobipolar transistor (hereinafter referred to as SiGe HBT) using silicon (Si) and germanium (Ge). As shown in FIG. 9A, the SiGe HBT according to the conventional embodiment has: a collector layer 102 composed of n-type silicon (Si); a base layer 103 composed of a p-type silicon germanium (SiGe); and an emitter layer 104 composed of n-type silicon (Si), which are formed successively on a semiconductor substrate 101 composed of p-type silicon.
The base layer 103 is formed to have a mesa configuration on the collector layer 102. A collector electrode 105 is disposed on the collector layer 102 to surround the base layer 103. The emitter layer 104 is also formed to have a mesa configuration on the base layer 103. A base electrode 106 is disposed on the base layer 103 to surround the emitter layer 104, while an emitter electrode 107 is disposed on the upper surface of the emitter layer 104.
As shown in the electron energy band diagram of FIG. 9B for the SiGe HBT according to the conventional embodiment, the emitter layer 104 is composed of silicon (Si) which is a semiconductor material having a larger band gap than a silicon germanium (SiGe) composing the base layer 103. Compared with a typical bipolar transistor composed of Si, the bipolar transistor according to the conventional embodiment can suppress the injection of holes from the base layer 103 to the emitter layer 104 so that the impurity concentration of the base layer 103 is increased. The arrangement increases the current gain of the transistor and reduces the sheet resistance of the base layer 103 so that a current-gain cutoff efficiency fT and a maximum oscillation frequency fmax are increased.
Since the SiGe HBT according to the conventional embodiment is a double heterobipolar transistor (DHBT) using Si also for the collector layer 102, the energy difference between respective valence bands in the collector and base layers 102 and 103 is increased. This increases the breakdown voltage of the transistor so that a transistor device with an excellent RF characteristic is provided.
However, since the conventional SiGe HBT uses silicon (Si) for each of the emitter and collector layers 104 and 102 and the silicon germanium (SiGe) for the base layer, the difference between the respective band gaps of Si and SiGe is about 0.5 eV, which is relatively small. Accordingly, the energy difference ΔEV between respective valence bands in an emitter/base junction portion and a collector/base junction portion is not sufficiently large. This causes the problem that the breakdown voltage and the current gain cannot be increased any more.